1. Field of the Invention
The present invention relates to a solid mesoporous crystalline composition of a pyrophosphate-phosphate of a tetravalent metal, characterized by a high surface area and a narrow distribution of mesopores, the process for its production and its uses.
2. Description of the Background
G. Alberti, S. Allulli, U. Costantino and N. Tomassini, in J. Inorg. Nucl. Chem., 40, 1113 (1978), described the production of lamellar phosphonates of tetravalent metals, with a structure similar to that of zirconium alpha-phosphate [.alpha.-Zr(HPO.sub.4).sub.2.H.sub.2 O], by reactions between phosphonic acids and the salts of tetravalent metals. These lamellar phosphonates can be represented by the general formula M(RPO.sub.3).sub.2, wherein M is a tetravalent metal and R an organic radical. A specific example is benzene zirconium phosphonate whose structure is shown in FIG. 1.
After this basic discovery, an intense research activity was carried out in this field, owing to the considerable applicative potentiality of the compounds obtained and in particular the following scientific patent literature can be mentioned: G. Alberti, U. Costantino and M. L. Luciani, J. Chromatog., 180, 45 (1979); G. Alberti and U. Costantino, J. Mol. Catal., 27, 235 (1984); G. Alberti, U. Costantino, J. Korney and M. L. Luciani, Reactive Polymers, 4, 1 (1985); G. Alberti, U. Costantino and G. Perego, J. Solid State Chem., 63, 455 (1986); EP 10.366; EP 10.857; M. B. Dines and P. M. Di Giacomo, Inorg. Chem. 20, 92 (1981); P. M. Di Giacomo and M. B. Dines, Polyhedron, 1, 61 (1982); M. B. Dines, P. M. Di Giacomo, K. P. Collahan, P. C. Griffith, R. H. Lane and R. E. Cooksey, A.C.S. Series 192, Chap.13, ACS, Washington DC, 1982; M. B. Dines, R. E. Cooksey, P. C. Griffith and R. H. Lane, Inorg. Chem. 22, 1003 (1983); M. B. Dines and P. C. Griffith, Polyhedron 2, 607 (1983); C. Y. Ortiz-Avila and A. Clearfield, Inorg. Chem. 24, 1773 (1985); A. Clearfield, Design of New Materials, Plenum Press, New York (1987), pages 121-134; and A. Clearfield, Chem. Rev. 88, 125 (1988).
EP 10.366 and EP 10.857 describe some compositions of the column or "pillared" type, obtained by reactions between salts of tetravalent metals and diphosphonic acids, with the formula M.sup.IV [R(PO.sub.3).sub.2 ], (wherein M is a tetravalent metal and R is a bivalent organic radical), whose structure is schematically illustrated in FIG. 2. In order to obtain compounds which can be used as molecular sieves, an attempt was made to introduce a certain degree of microporosity between the layers; a proposal was then made to dilute the columns in the interlayer zone, by partially substituting them with very small groups of the type R'PO.sub.3, such as for example phosphite, HPO.sub.3 groups. EP 492/694 describes the preparation of a diphosphonate-phosphite of a tetravalent metal having a high degree of crystallinity and a high degree of porosity which can be attributed to microcavities in the interlayer region induced by the presence of regularly distributed phosphite groups.
U.S. Pat. No. 5,166,380, as well as Italian Patent applications 95/A000710 and 96A/001106, subsequently disclosed that it is possible to prepare a diphosphonate-phosphite of a tetravalent metal, in crystalline form, under such conditions that the HPO.sub.3 groups, in addition to the formation or non-formation of micropores in the interlayer region, also induce the formation of mesoporosity, which cannot be attributed to the formation of cavities in the interlayer region, characterized by a rather narrow distribution of the pore dimensions, most of which have dimensions within the range of 20-30 .ANG..